O-8 Association of MSS/MSI-RNF43 molecular subtypes and clinical outcome of metastatic BRAF-V600E colorectal cancer patients treated with anti-BRAF/EGFR ± combinatorial therapies
Authors: E. Elez, J. Ros, G. Randon, S. Lonardi, C. Cremolini, F. Pietrantonio, J. Tabernero, R. Toledo
Journal: Annals of Oncology (2022)

The BEACON trial (NCT02928224) applied BRAF combinatorial blockage (encorafenib/cetuximab ± binimetinib) in the phase-III setting and showed improved ORR, progression-free survival (PFS), overall survival (OS) for the first time in this patient population⁠. Based on these results, the anti-BRAF/EGFR combinatorial therapy was recently approved by the FDA and EMA for the treatment of metastatic BRAF-V600E colorectal cancer (CRCBRAF-V600E). However, the underlying genetic determinants of its anti-tumoral efficacy remain unknown. We performed a multicentric discovery-validation study and discovered for the first time a strong association between MSS/MSI-RNF43 molecular subtypes and the efficacy and clinical outcome of CRCBRAF-V600E patients treated with anti-BRAF/EGFR ± combinatorial therapies. Discovery and validation patient cohorts: A total of 98 metastatic CRCBRAF-V600E patients treated with anti-BRAF/EGFR ± combinatorial therapies in clinical trials, or who received such therapies through a compassionate use program, were included in the current study. The discovery cohort was composed of 46 patients from the Vall d'Hebron University Hospital (Barcelona, Spain) prospectively included from 2013 to 2021, and the validation cohort comprised 52 patients from three academic hospitals from Italy (Fondazione IRCCS Istituto Nazionale dei Tumori, Milan, Italy; University Hospital of Pisa, Pisa, Italy; Istituto Oncologico Veneto IOV-IRCCS). Biomarker discover: To determine which genes were enriched for somatic mutations in responder and non-responder groups, we used the dNdScv method to compute somatic enrichment p- and q-values for each group based on whole-exome sequencing (WES) and Next-Generation Sequencing (NGS) cancer gene panel data. To further demonstrate the biological impact of the identified RNF43 missense mutations, we performed luciferase reporter assays with ectopic expression of the RNF43 mutants and immunohistochemical analysis. Using data from the discovery cohort, we discovered that patients with MSS-RNF43-mutated tumors achieved a higher ORR in comparison with MSI-RNF43-mutated and MSS-RNF43-wild-type (72.7%, 50% and 30.8%, respectively). Multivariate analysis of the external validation cohort (N=52) showed improved PFS and OS of MSS-RNF43-mutated in comparison with MSS-RNF43-wild-type (HR 0.30, 95% CI 0.12-0.75; p=0.01; and HR 0.26, 95% CI 0.10-0.71; p=0.008, respectively). Mechanistically, our in vitro experiments showed that the RNF43 mutations from responders were loss-of-function, leading to high signal levels on the luciferase reporter assay. In summary, this study shows that the MSI/MSS-RNF43 molecular subtypes emerge as a potential stratification biomarker for CRCBRAF-V600E patients treated with anti-BRAF/EGFR ± combinatorial therapies and uncovers the crosstalk between the MAPK and WNT/β-catenin pathways as a new therapeutic avenue to be exploited in this setting.